A method of this type and a casting mold for executing it is known from East German Letters Patent 257 350, PFANNKUCHEN et al., together with East German Letters Patent 207 076, KRUMPHOLD et al., cited therein as prior art. A method for producing round disks of metal silicides with a diameter of 156 mm and a disk thickness of 8 mm is described in East German Letters Patent 207 076. In this case, a melt of a Cr-Si-W alloy is poured into a graphite mold preheated to.gtoreq.700.degree. Celsius and thermally insulated on the outside and is evenly cooled to room temperature in a vacuum at a cooling rate of less than 20.degree. C./min.
This method is well suited for producing thin disks; however, with cast bodies of larger wall thickness, cracks and bubbles appear in spite of preheating the mold and slow cooling. These can be caused, for example, by the unfavorable cast texture of the cast body, by a collection of deleterious precipitates, segregation, or pores in the center of the cast body or by the contraction of the cast body during cooling being impeded because of inhomogeneities on the interior walls of the casting mold.
To overcome these disadvantages, a cylindrical casting mold is proposed in East German Letters Patent 257 350, with a soft insulation layer glued to its inside, which does not offer great resistance to the contraction of the cast body, and into which a metal bottom plate of good thermal-conducting properties and of the same chemical composition as the material to be cast has been inserted. A directed solidification of the melt is attained by means of the specific thermal dissipation from the melt via the bottom plate in such a way that only a single solid-liquid interface is formed between the already solidified material and the molten material which, starting at the bottom of the casting mold moves essentially parallel to the bottom in the direction of the exposed melt surface in the course of continued solidification of the melt.
It is known from the publication "Gerichtete Erstarrung" [Directed Solidification], W. Kurz and B. Lux, Zeitschrift fur Metallkunde, Vol. 63, No. 9, pages 509 to 515 (1972), that such directed solidification can bring advantages regarding the segregation, secretion and bubble reactions of cast bodies. It is also known that directed solidification can cause cleaning of the cast body, in that the solid-liquid interface moving from the bottom of the casting mold in the direction of the exposed surface pushes foreign material, which is hard to dissolve in the solidified material, ahead of itself up to the surface of the melt. In this way, the foreign materials are concentrated at one end of the cast body, where they are less harmful in terms of the solidity properties of the cast body, and can be easily removed, if required. In the known processes for directed solidification, solidification of the material melts proceeds very slowly in order to keep the stresses generated in the cast body by casting and the subsequent cooling process small or to reduce them and to make the control of the specific, directed solidification easier. This is attained, for example, in that the casting mold is pre-heated before the melt is poured in and thereafter is cooled slowly and evenly. For example, a cooling speed to room temperature of less than 20.degree. C./min is recited in East German Patent 207 076.
It is known from German Published, Non-Examined Patent Application DE-OS 35 32 131, SWIRTLICH et al., to maintain a temperature gradient over the height of the sidewalls of the casting mold, where the temperature at the upper edge of the casting mold lies in the range of the melting temperature of the material to be cast. By means of this, an exact control of the directed solidification of the melt is assured, starting at the bottom, with good thermal dissipation up to the upper edge of the casting mold. The melt solidifies very slowly in this case. In DE-OS 35 32 131, the speed, at which the solid-liquid interface proceeds, is stated to be 4 cm/h.
Depending on the material to be cast, a relatively large-grained structure is created by slow solidification and cooling which can also be a cause of the formation of cracks in the cast body. The force necessary for generating cracks is essentially dependent on the atomic bonds and the microstructure of the material. In connection with polycrystalline materials, grain boundaries can be regarded as intrinsically present incipient cracks, starting from which the spreading of cracks is facilitated. This property of grain boundaries of triggering cracks becomes more marked with lengthening of the individual grain boundaries, i.e. with a coarser grain structure of the material. In contrast thereto, the triggering of cracks or the spreading of cracks is hampered by fine-grained structures.
In addition, slow cooling can also aid the generation or growth of undesirable inhomogeneities, such as secretions or dissociations, in many materials susceptible to this, which results in fluctuations in the coating results when the material is used as a target for coating purposes, for example. Inhomogeneities of this type in the structure of the material can also encourage cracking.
Gaseous impurities, such as water vapor or oxygen, diffuse in larger amounts into the melt over the exposed melt surface, as well as the interior walls of the casting mold, because of the slow solidification of the melt, where they not only represent contamination of the material in the form of foreign materials, but can also act as nuclei for inhomogeneities forming in the material.
To avoid or reduce crack formation in the cast body, the use of a bottom plate with the same chemical composition as that of the metallic material to be cast is proposed in East German 257 350. A similar suggestion is also the basis of the method in accordance with European Patent Disclosure EP-B1 237 325, and corresponding U.S. Pat. Nos. 4,739,818 and 4,824,735, McGILL, where a bottom plate of a material is used which combines with the material to be cast to form a unified structure and which has a lesser expansion coefficient than that of the material to be cast. The surface of the cast body is placed under compressive strains because of this which, although they can prevent the spreading of thermal cracks over the entire thickness of the cast body, cannot prevent the generation of cracks.
Aside from the fact that, when bottom plates of the same chemical composition as that of the material to be cast are used, the thermal conductivity of the bottom plate cannot be optimized, there is also the danger of tearing of the bottom plate in the case of materials susceptible to thermal cracking because of the thermal stress when the hot melt is poured over it. With the use of bottom plates of a composition differing from that of the material to be cast, which are intended to be firmly connected with the latter, there are not only possible undesirable boundary reactions and adhesion problems, but also deformations of the cast bodies because of the different thermal expansion coefficients of the two material connected with each other, which can also result in problems when the cast bodies are brought to their intended use.
To produce disks of the material susceptible to thermal cracking, for example with the use as targets for coating purposes, a cylindrically shaped base body, such as produced by means of the casting mold in accordance with East German Patent 257 350, must be cut into appropriate disks or must be divided in some other manner. The material removed in the course of this, as well as the additional rejects as a result of the stress on the cast body during working, inevitably result in losses of material.